Our modern day civilization depends heavily upon a supply of electrical energy to perform a multitude of tasks. The supply of electrical energy, however, must be controlled, and in many cases instantaneously. One method of controlling the supply of electrical energy is through the use of opening electrical contacts or make-and-break contact devices.
Make-and-break contact devices are designed for and used in a variety of forms for various applications. These devices may be small switches or relays which are actuated by tiny forces and which carry small currents, or they may comprise large units requiring great forces to actuate them and which are capable of carrying enough current to light a city. Similarly, the actual contacts incorporated into these devices may vary from small rivets of pin-head size to large contactors comprising several pounds of material.
Conventional make-and-break contact devices, under normal current conditions, facilitate electrical connection between two contacts by bringing them into physical contact such that an ohmic connection is made. Under fault current conditions, conventional make-and-break contact devices move the contacts apart breaking the ohmic connection.
In conventional make-and-break contact devices, there are two forces that operate on the contacts during a fault current condition, namely a magnetic repulsion and a gas force. Specifically, when a conventional make-and-break contact device is subjected to a fault current, a magnetic repulsion force is created by the flow of current through the contacts. This magnetic repulsion force acts to separate the contacts. Typically, contact pairs will consist of a spring-loaded moving contact and a stationary contact. Under normal current conditions, the spring force exerted on the contacts will be much greater than the magnetic repulsion force generated by the flow of electricity through the contacts. Notwithstanding, there is a current at which the magnetic repulsion force will match the spring force. When the current exceeds this value, the contacts will part.
If the current and voltage are high enough, an arc will form between the contacts. Once the contacts separate and arcing commences, a second force, namely the gas force, will act upon the contacts. This gas force derives from the ionization of the air and other materials in the arc chamber which temporarily increases the pressure between the contacts.
In conventional make-and-break contact devices, the contacts may be operated many times without appreciable wear provided no current is being carried. When current is being interrupted, however, significant changes in the contact surfaces may occur. Generally, arcing will cause the transfer of material from the negative contact to the positive contact. Also, oxidation of the contact surfaces may result from the energy expended between the contacts during arcing. These changes in the contact surfaces will eventually affect the reliability of the make-and-break device requiring its replacement.
In addition, as long as an arc bridges the gap between separating contacts, current will continue to flow through the make-and-break device. This continued flow of current is termed a let-through current. As long as the voltage across the contacts exceeds the arc voltage, the arc will continue to burn and current will continue to flow though the device.
What is needed are make-and-break contact devices which inhibit arcing associated with conventional devices. What is also needed are make-and-break contact devices which rapidly open the circuit and inhibit let-through currents.